1. Field of the Invention
The invention relates to techniques for annealing a nonferrous, metallic workpiece and, more particularly, to techniques for annealing a selected portion of a nonferrous, metallic workpiece, utilizing a laser.
2. Description of the Prior Art
It is often necessary that a nonferrous, metallic member have different physical properties in different portions of the member. Phosphor-bronze or beryllium-copper connector contact springs, for example, must be hardened to spring or extra-spring hardness in order to perform their basic function, i.e., the making and maintaining of good electrical connections. Such spring members must often, however, be joined to circuit paths on a brittle substrate, e.g., by thermocompression bonding. In order that thermocompression bonding may take place, the metal in the bonding area of a spring member which is to be bonded to a circuit path must be relatively soft so that the bond can be effected without cracking the brittle substrate.
At present, dual metal connector contact springs are employed to provide the different physical properties required for good electrical contacting and good thermocompression bonding capabilities. Thus, composite metal rolling operations may provide beryllium-copper alloy and copper spring members, the beryllium-copper alloy component being hardened to the necessary degree for the spring members to function properly, and the copper component being sufficiently soft to permit thermocompression bonding of the spring members to the circuit paths. Such composite spring members, while effective to provide the required properties, are quite costly to manufacture.
A technique for treating a single component, nonferrous, hard spring member, in order to soften the material of the spring member in only a small, locallized bonding area, might involve the annealing of the spring member at only the bonding area. Utilizing a furnace, for example, complex masking fixtures might be employed to shield the spring in other than the bonding area. However, for nonferrous metals at spring or extra-spring hardness, the transition from the hard to the fully annealed state is so rapid that it is not possible to obtain consistently a required intermediate value of hardness in mass production. The presence of a fully annealed region on a spring member is considered disadvantageous since, for example, the spring member would be subject to distortion in handling.
It is known to employ a continuous wave laser to heat soften a metallic workpiece. The continuous wave laser, heat softening technique, however, requires the continuous application of a relatively low level of power to the workpiece for a relatively long period of time. Thus, lateral conduction of heat within the workpiece during treatment with a continuous wave laser makes controlled, localized heating of only a selected portion of the workpiece virtually impossible.
It is also known to shock harden a selected surface area of a metallic workpiece, which may be a ferrous workpiece, by employing a pulsed laser. Such localized shock heating by a pulsed laser typically requires the application of very high energy density levels to the selected surface area, typically, through a surface coating or overlay.